In recent years, energy harvesting in which electric power is obtained by capturing ambient energy is attracting attention. The ambient energy includes, for example, sunlight, illumination light, electromagnetic waves, and vibrations and heat from machines and human bodies.
In order to use electric power derived from such energy harvesting as a power source for a load circuit, a DC-DC converter is used, for example. The DC-DC converter raises a voltage (an ultra-low voltage of tens of mV to hundreds of mV, for example) obtained from an ultra-low voltage generation unit (a photovoltaic generator, an electromagnetic generator, a vibration power generator, a thermoelectric generator, or the like).
Here, at the time of starting the DC-DC converter (step-up or boost converter), for example, the ultra-low voltage is oscillated by an oscillator to generate an alternating sine wave signal (alternating current signal) with a sufficient amplitude. Then, a switching transistor in the step-up converter is driven by the alternating current signal. Conventionally, various types of boost converters have been proposed. Related-art techniques are disclosed in Japanese Laid-open Patent Publication Nos. 2000-116136, 58-175975, and 2000-166244.
As described above, at the time of starting a boost converter, for example, the switching transistor of the boost converter is driven using an alternating current signal generated from an oscillator. However, driven by an alternating current signal (alternating sine wave signal), the transistor cannot switch rapidly, which may lead to a low power efficiency, or the like.
Thus, we have proposed to convert the alternating current signal generated from the oscillator into a pulse signal, and to drive the switching transistor by the pulse signal. In a signal conversion circuit that converts such an alternating current signal into a pulse signal, low power consumption is demanded. However, currently, a signal conversion circuit that fully meets the low-power requirements has not yet been put into practical use.
In this regard, a description will be given mainly based on the application to energy harvesting in this specification. However, a signal conversion circuit and a power supply apparatus according to the present embodiments are not limited to the application to energy harvesting, as a matter of course.